1. Field of the Invention
The present invention relates to a power supply circuit for any kind of loads or power consuming devices which are connected to an electrical power supply; especially lamps, such as at least one glow or gas discharge lamp, wherein a pulse width modulator controls electronic switches of a push-pull oscillator, and wherein the pulse frequency of the pulse width modulator is tuned to the resonant frequency of a resonance transmitter of the push-pull oscillator which has the secondary side thereof connected to the gas discharge lamp.
2. Discussion of the Prior Art
A power supply circuit of that type for a gas discharge lamp is described in the specification of German Laid-Open Patent Appln. 40 05 776 A1. In order to be able to attain a high degree of efficiency, the frequency of the current flowing through the lamp is adjusted by the pulse frequency of the pulse width modulator to the resonant frequency of an oscillating circuit of the resonance transformer. Hereby, the resonance frequency is independent of that of the power supply.
The pulse frequency of the pulse width modulator is thusly tuned or adapted through the intermediary of a variable impedance or resistance to the resonance frequency of an oscillating circuit formed from the secondary winding of the resonance transformer, a capacitance and the lamp. However, when the resonance frequency changes, there is then encountered a mistuning which places into question the attainment of the desired high degree of operating efficiency. Such a change in the resonance frequency can be encountered, for example, through the ageing of the components or in response to temperature changes. Moreover, in the utilization of different gas discharge lamps possessing differing resonance frequencies, it is possible that in German 40 05 776 A1, in every individual instance there would be required a correlation of the pulse frequency of the pulse width modulator to the current resonance frequency.
In the disclosure of German 40 05 776 A1 there is proposed that for the dimming of the gas discharge lamp, there is correspondingly adjusted or set the keying ratio of the pulse width modulator.
In the disclosure of German 40 05 776 A1, by means of a transistor of a rectifying bridge there can be controlled the preheating of the electrodes of the gas discharge lamp.
Circuit arrangements in which the gas discharge lamp are operated at the frequency of an alternating current power supply are set forth in the disclosures of German Patent Publications 33 27 189 A1 and 25 12 918 B2. In those instances, it is not contemplated to connect the gas discharge lamps to a direct-current voltage supply, or to operate the gas discharge lamps at a frequency which is higher than the frequency of the alternating-current power supply which powers the lamps.